AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
A nucleotide is made up from a phosphate ion bonded to 2-deoxyribose which…
Full specification wording
DNA (A-level only): A nucleotide is made up from a phosphate ion bonded to 2-deoxyribose which is in turn bonded to one of the four bases adenine, cytosine, guanine and thymine.
The unit is built in two condensation steps, and both eliminate water.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The unit is built in two condensation steps, and both eliminate water. The phosphate joins through the CH₂OH group on carbon five of the sugar, giving a phosphate ester link. The base joins at the far end, at carbon one, through a nitrogen atom that carries a hydrogen in the booklet structure; that N-H and the sugar's OH condense together. So the sugar sits in the middle, phosphate hangs off one end and the base off the other, and phosphate is never bonded straight to base. Each unit carries exactly one base, which is why there are four of them in DNA and why they differ in nothing else. Adenine and guanine are the two-ring purines; cytosine and thymine are the single-ring pyrimidines.
What earns the marks
- Naming the three components as a phosphate ion, 2-deoxyribose and one of the four bases.
- Showing the phosphate bonded to the sugar through an oxygen atom.
- Showing the base bonded to the sugar through a nitrogen atom.
- Identifying both linkages as condensation reactions, with water eliminated at each.
Common mistakes and how to put them right
Bonding the base straight to the phosphate, which leaves the sugar dangling off the side.
Attaching the base through a carbon atom instead of through nitrogen.
Showing two or three bases attached to one sugar.
Forgetting that each link is a condensation, so water is never shown as a product.
Drawing ribose, which belongs to RNA, in place of 2-deoxyribose.
Exam tips
- Say condensation and show the water; the elimination is often a mark in its own right.
- Draw the sugar first and in the middle, because students who start from the phosphate run out of room and blur the linkage.
- A structure may be displayed, structural or skeletal provided it is unambiguous, so pick the style that shows the two new bonds most clearly.
Try the lesson
The start of the DNA (A-level only): A nucleotide is made up from a phosphate ion bonded to 2-deoxyribose which is in turn bonded to one of the four bases adenine, cytosine, guanine and thymine. lesson, made from checked teaching notes for this specification point.
Have a go first
Which sugar is bonded to the phosphate ion in a DNA nucleotide?
Listen
The unit is built in two condensation steps, and both eliminate water. The phosphate joins through the CH₂OH group on carbon five of the sugar, giving a phosphate ester link. The base joins at the far end, at carbon one, through a nitrogen atom that carries a hydrogen in the booklet structure; that N-H and the sugar's OH condense together. So the sugar sits in the middle, phosphate hangs off one end and the base off the other, and phosphate is never bonded straight to base. Each unit carries exactly one base, which is why there are four of them in DNA and why they differ in nothing else. Adenine and guanine are the two-ring purines; cytosine and thymine are the single-ring pyrimidines. A strong answer earns marks for these points. naming the three components as a phosphate ion, 2-deoxyribose and one of the four bases. showing the phosphate bonded to the sugar through an oxygen atom. showing the base bonded to the sugar through a nitrogen atom. identifying both linkages as condensation reactions, with water eliminated at each. Exam tip: Say condensation and show the water; the elimination is often a mark in its own right.
See it
Key words
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phosphate ester link
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purines and pyrimidines
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base to sugar bond
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See every definition
- phosphate ester link
- The link formed when the phosphate joins through the CH₂OH group on carbon five of the sugar.
- purines and pyrimidines
- Adenine and guanine are the two-ring purines; cytosine and thymine are the single-ring pyrimidines.
- base to sugar bond
- The base joins at carbon one through a nitrogen atom, and phosphate is never bonded straight to base.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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